The present invention relates to a method of operating a laser source to effect dye thermal transfer printing. There are three main types of dye thermal transfer printing methods, in which dye is transferred by melting, diffusion and sublimation respectively.
In the diffusion method, a dye donor ribbon and a dye receiver ribbon, comprising a dye layer and a receiver layer, respectively, on a supporting substrate, are held in contact with one another, and a localized source of energy is used to heat selected pixel regions of the dye layer to cause dye in those regions to become thermally mobile and diffuse into the adjacent receiver layer to produce a pattern of printed pixels therein. A desired print may be produced by heating an appropriate selection of pixel regions in the dye layer. By applying more or less energy to a pixel region of the dye layer, more or less dye is transferred to the receiver layer, and so darker or lighter printed pixels are produced. This allows for continuous tone printing.
Laser sources are often selected as the energy source, because they can provide an intense, highly directional and controllable output. When laser sources are selected, laser light absorbing material is normally provided in the dye ribbon, either as a separate layer or dispersed within the dye layer, to convert the laser energy to heat.
Typically, the output of a laser source is scanned across the donor ribbon at a set speed, and the laser source output is pulsed on and off. A heated pixel region is produced in the dye layer whenever the output is pulsed on, and the darkness of a printed pixel depends upon the amount of dye transferred to it from the corresponding heated pixel region in the dye layer, which in turn, depends upon the power and length of the laser pulse applied to that pixel region. The spacing between adjacent printed pixels depends upon the scan rate of the laser output across the dye layer and on the time between the start of the pulses producing the printed pixels.
A region of darkness may be produced in a print by printing a line of adjacent dark pixels, and it is known to select the scan rate and laser pulse rate so that each printed pixel partly overlaps those adjacent it in the scan direction. This ensures that the printed dark region is of constant optical density, and compensates for the fact that each printed pixel is slightly darker at its centre than at its periphery due to the gaussian cross-sectional profile of a typical laser output. The overlap generally coincides with the half width points of the laser beam profile (measured at l/e of maximum profile intensity) when projected onto the printed pixels.